The present invention relates to a control unit for controlling an inverter having a bridge circuit provided with controllable semiconductor switches, which bridge circuit converts a direct voltage fed to the inverter into an alternating voltage of variable frequency and amplitude, said control unit controlling the semiconductor switches by pulse-width modulation (PWM) to produce an output voltage of the inverter.
The invention relates especially to a PWM inverter control arrangement which comprises a control circuit at ground potential used e.g. for a customer interface and a control circuit at main circuit potential, containing e.g. measuring circuits and circuits producing control signals for power semiconductor switches.
Such an inverter is applicable for use e.g. in a frequency converter in which the alternating voltage of a three-phase supply network is rectified by a rectifier circuit to produce an intermediate circuit direct voltage and then inverted by means of an inverter to produce a three-phase alternating voltage of variable frequency and amplitude for feeding e.g. a three-phase cage induction motor.
PWM frequency converters are generally used to control the rotational speed of a cage induction motor, which is the commonest motor type in industry. An output voltage of variable amplitude and frequency is generated by means of the power semiconductor switches of an inverter. For the control of the power semiconductor switches, pulse amplifier circuits at main circuit potential, i.e. at the potential of the power semiconductor switches are needed. To produce a correct output voltage, it is necessary to use measuring circuits, which are normally also at main circuit potential.
The control circuit processing the signals pertaining to the customer interface again has to be at ground potential for reasons of safety. Therefore, galvanic isolation is required between the control circuit unit at ground potential and the control circuits at main circuit potential.
A common practice for providing galvanic isolation between the units is to use serial communication via optoisolators or optical fibers. Such an arrangement is known e.g. from patent specification EP0469872, which discloses a system for the control of an adjustable-frequency AC inverter using two serial data channels: one channel for transmitting reference values to a control circuit at main circuit potential and a second channel for transmitting measurement and other feedback data to a control circuit at ground potential.
The pulse pattern of the voltage produced by the inverter is normally computed either by a microprocessor or an ASIC circuit designed for this purpose. The generation of the pulse pattern is normally based on a predetermined algorithm, e.g. a sine-triangle comparison. The modern efficient microprocessors allow both the functions required for the control of the customer interface and the functions needed for controlling the main circuit to be performed by the same processor, which is normally placed in a the control unit section at ground potential.
The generation of a pulse pattern implemented in the aforesaid manner involves the problem that the pulse pattern actually produced in the output voltage is not exactly of the desired form. Deviations arise e.g. from signal processing delays in the pulse amplifiers, switching delays in the power semiconductor switches and the required so-called dead time between the conduction periods of the upper and lower arms of the phase switch. Further errors result from a fast variation of the d.c. intermediate circuit voltage. Especially at low frequencies, at which a large number of switching actions occur during a single cycle of the output voltage, the errors clearly distort the output voltage.
The voltage error can be compensated by measuring the output voltage produced and taking the error into account in connection with the next switching instant. Especially if the pulse pattern is generated at ground potential, the large voltage difference requires the use of difficult and expensive measuring circuits which reduce the ground impedance of the device and may cause problems in the earth-leakage monitoring circuits of the supply network.
The object of the present invention is to achieve a control system for the control of a PWM-controlled inverter that is better than prior-art solutions, in which system the output voltage is always correct regardless of frequency, power-stage dead time or variations of the intermediate circuit voltage, wherein the phase switches do not have to perform any unnecessary actions, the switching losses being thereby reduced, wherein the angle of the output voltage vector is continuously known, allowing easier motor adjustment especially in difficult applications, and wherein the distribution of tasks between the control unit at ground potential and the control unit at main circuit potential is natural, so that no difficult main circuit measurements are needed and no ground impedance problems caused by them arise.
In the control system of the invention, this is achieved by calculating in the control unit section at ground potential the instantaneous value of the voltage vector of the output phases, which value is transmitted in serial format over a galvanically isolated channel to the control unit section at main circuit potential, where the modulation, i.e. determination of the phase switch positions is carried out by forming the time integral of the difference between the reference and actual values of the phase voltages, comparing this difference to a positive limit and a negative limit, and if the time integral exceeds the positive or the negative limit, the phase switch is turned accordingly either to the up position or to the down position.
In an embodiment of the control system of the invention, the angle and amplitude of the output voltage vector is given in the channel between the galvanically isolated control circuits.
In a second embodiment of the control system of the invention, the reference values of the instantaneous voltages of at least two phase voltages are given in the channel between the galvanically isolated control circuits.
The features of the invention are presented in detail in the claims below.